This is soooo long so apologies!
When traffic is merging from the left, if there's space I always move over one lane to the right - this gives more space for merging traffic to get in, you can usually maintain a smooth speed and don't have to brake because the vehicles merging are likely travelling at a slower speed. Once the junction is passed and it's safe to do so I move left again.
If you absolutely hate it then there's no shame in using A roads for a portion - but it does add on loads of time and your sat nav will hate you.
Sunday will be much quieter on the motorways. Less commercial vehicles.
You need to be looking way ahead and looking for brake lights, slow moving traffic and merging lanes. You also need to looking in all your mirrors frequently as cars can come charging up behind and if you want to change lanes you need to know exactly where the other cars behind, in front and both sides are.
If you accidentally come off at the wrong junction then don't panic.
If you have passengers then don't hesitate to tell them to shut up - eg if traffic is heavy, weather conditions, tricky road navigation etc etc.
If it all gets a bit hairy and nerve wracking then come off at services and have a bit of a regroup.
Do not use phone (even hands free), change sat nav, scrabble around for snacks, or alter radio/heater controls etc - get passenger to do that. Keep eyes on the road. Pull in at services if you need to reprogramme satnav etc.
Get passengers to watch out for road signs as well.
When you come off the motorway then there's a countdown at the side of the road, try and get into the lane asap, even if it means pootling along for a mile or so, so much easier than trying to get into a left hand lane and not being able to because there's too much traffic.
I don't do a large amount of driving on motorways, but find it's not hideous. They are just a bit boring to drive on, especially if there are traffic queues, or even little traffic, but ever so useful to get quickly from A to B.
You will be fine. Enjoy the experience!